1. Field of the Invention
The invention relates generally to a motor control unit and an electric pump unit, and more specifically to a motor control unit for an electric pump unit suitable for supplying hydraulic pressure to a transmission of an automobile and an electric pump unit that includes such a motor control unit.
2. Description of Related Art
As a device that supplies hydraulic pressure to a transmission of an automobile, a device that includes only a main pump driven by an engine that is a main power supply has been used conventionally.
However, when an idling stop function of stopping the engine while the automobile stops is provided, two hydraulic sources, that is, a main pump, which has been used conventionally, and an auxiliary pump driven by an electric motor that uses a battery as a power supply are required to secure supply of hydraulic pressure to a drive system such as the transmission even while the engine is stopped by the idling stop function. Japanese Patent Application Publication No. 2010-116914 (JP 2010-116914 A) describes an example of a hydraulic pressure supply device for a transmission, which includes such two hydraulic sources. The hydraulic pressure supply device supplies hydraulic pressure to a transmission, and an auxiliary pump constitutes an electric pump unit together with an electric motor for driving the auxiliary pump and a motor control unit. When the hydraulic pressure in a main discharge oil passage from a main pump to the transmission is equal to or higher than a predetermined value, driving of the auxiliary pump is stopped. On the other hand, when the hydraulic pressure in the main discharge oil passage is lower than the predetermined value, the auxiliary pump is driven. The hydraulic pressure that is supplied from the main pump is several tens of times higher than the hydraulic pressure that is supplied from the auxiliary pump, and the measurement range of a hydraulic sensor is set based on the magnitude of the hydraulic pressure that is supplied from the main pump. Therefore, if the hydraulic pressure that is supplied from the main pump is used for hydraulic control of the auxiliary pump, measurement accuracy is not sufficient, and it is difficult to execute the hydraulic control. When the auxiliary pump is driven, the electric motor is driven based on a current command value provided from a host ECU. Therefore, it is possible to obtain hydraulic pressure that is equal to or higher than a target hydraulic pressure.
In the conventional electric pump unit, the electric motor is driven according to the current command value regardless of whether there is any load on the electric motor or not, which causes a state where an output is large beyond necessity (over power). Such a state is not preferable because power consumption is unnecessarily large and heat and noise are generated. The target hydraulic pressure needs to be set in order to suppress over power. However, in order to supply a wide range of hydraulic pressures by a single pump, how to deal with changes in the load on the pump is an issue to be addressed.